1. Technical Field
This invention relates to the art of assembling precision fitting parts; and, more particularly, it relates to assembly of valve spools into valve openings of an automotive automatic transmission control unit.
2. Discussion of the Prior Art
Automatic mechanical assembly of precision fitting parts requires a method to accurately locate the parts relative to each other so that mating or interfit takes place without interference or contact. One example is the insertion of precisely ground valve spools into an opening having a series of sequentially reducing land diameters within the opening, both the lands and the valve spools being sharp-edged or sharp-shouldered with no chamfer or lead-in edges; such valve spools are used in automatic transmission control units. The valve spools are designed to have a diametrical clearance with the lands of the opening of about 0.0003 inches or less; this is extremely close-fitting. When the valve body is machined to define the land surfaces, the process tolerances of boring may allow the opening location to vary from part to Part as well as permit several of the openings to vary in location within a single part. Such variations often are greater than the designed valve to opening clearance. These transmission control units and valve bodies can reach extremely complex proportions, often having 16 valve openings distributed on several different sides of the control unit. For such complex control units to function properly, with complex fluid pressures communicated throughout its labyrinth of passages, requires careful assembly to assure the valve spools and springs are seated correctly without damage, such as scratching, galling or burrs. Such accurate assembly within a tolerance of 0.0003" is not within the capability of state-of-the-art robots.
Efforts to automate such assembly, which are known to the inventors herein, comprise the use of compliant devices, slave plugs, and vision sensors. Compliant devices require lead-in edges on the valve opening or valve spool to permit off-center mating of the parts; the device responds to side thrust to provide centering. The requirement for lead-in edges frustrates the design of the control unit and must be carried out at slow speeds to avoid damage to the parts during off-center mating.
Slave plugs necessitate extra strokes of the assembly due to the need to stroke the parts together using the slave plug for alignment and again when the actual valve spool is inserted. Due to the possible contact between the slave plug and part, the speed of such assembly method must be undesirably slow to avoid damage.
Vision or laser sensors often use search routines that comprise trial and error to find the correct alignment. The insertion carriage travels to where it believes the opening resides according to a hunt routine. This is not only time-consuming, expensive and complicated, but also is not reliable in precisely locating the opening for precise noncontact alignment.
It is therefore an object of this invention to provide a method and apparatus for locating close-fitting parts within a receptacle which obviates the above problems of the prior art, and additionally is characterized by rapid, simple, reliable and microscale accuracy in carrying out such method. It is also an object of this invention to provide such a method which is adaptable to different types of close-fitting parts for reliable automatic assembly.